The newer versions of Clojure have moved to 64-bit arithmetic for good, which is why `int` is giving speedup in 1.2 for you. Now, if your post was annotated with line numbers I could mention those in my comment. Another thing to note is when you run the code with JVM arg "-server" it automatically JIT-inlines functions doing primitive arithmetic stuff, so let the JVM warm up for a while before measuring anything. Criterium is the right tool. Nice run-through though.